1. Field of the Invention
This invention relates to the display of information on a cathode ray tube (CRT) monitor display. In particular, the invention pertains to apparatus which permits the display controller which generates the dot pattern comprising the characters of information to be displayed on the CRT screen to generate and modify the video control signals such that the normal video output can be modified by certain visual attributes associated with the information to be displayed.
2. Description of the Prior Art
Information is normally displayed on the cathode ray tube of a display monitor by selectively energizing an electron beam as it scans the sensitized screen of the CRT. The electron beam normally scans the screen from left to right in a succession of horizontal scan paths which begin at the top of the screen and end at the bottom of the screen. The beam is subsequently returned to the top of the screen for the next successive raster scan of the entire screen. This is accomplished by monitor electronics, or beam drive circuitry, associated with the cathode ray tube which magnetically deflects the beam in both the horizontal and vertical directions and selectively energizes the beam as it scans the screen of the CRT. The horizontal retrace of the beam is initiated by a horizontal synchronization (SYNC) signal, the vertical return of the beam to the top of the screen is initiated by a vertical sync signal and the beam is selectively energized in response to a video signal. These signals, the horizontal sync, vertical sync, and video signals are generated by the display controller and transferred to the monitor electronics which in turn uses them to generate the signals which drive the electron beam gun and beam deflection magnets.
The display controller generates the horizontal sync and the vertical sync signals by use of raster scan logic. The video signals are generated by scanning a refresh memory in the display controller which contains the information which is to be displayed on the CRT screen. The video signals are generated by the display controller scanning the refresh memory a character at a time as each row of information is displayed on the CRT screen. The information within the display controller refresh memory may originate from a keyboard attached to the display terminal, from a computer attached to the display controller, or remotely from a communications line attached to the display controller.
In addition to generating the video and sync signals, some displays allow the information to be displayed in a variety of intensities on the CRT screen, for example, a display may allow information to be displayed in normal brightness or in a low intensity mode which is less than the normal brightness. In this case, a low intensity signal must also be generated by the display controller to control the intensity of the information on the display screen. In addition to an intensity mode which may be associated with an individual character or a field of characters which is to be displayed on the display screen, other visual attributes are often found in display systems. For example, an inverse video attribute can indicate that the character of information is to be displayed as a dark character on a light background as opposed to the normal case of a light character on a dark background. A blink video attribute allows the character of information to be blinked on the display screen to draw the display operator's attention to the information. An underline visual attribute allows the character of information in the row to be displayed with an underline under the character. A hide visual attribute results in the blocking of the video signal such that sensitive data will not be displayed on the display screen, although it is available in the refresh memory and may be transmitted or received from a computer attached to the display controller or remotely over a communication line attached to the display controller. In addition, the cursor may be treated as a visual attribute to modify the character which would otherwise be displayed on the display screen to indicate to the operator where the next character of data which is entered from a keyboard attached to the display controller will be placed on the display screen.
Because one or more of these visual attributes may be associated with any character of information to be displayed on the display screen, a large amount of combinational logic is needed to combine the various visual attributes to modify the video signal prior to it being transmitted to the CRT's electron beam gun. Besides requiring large amounts of combinational logic if many visual attributes can be associated with any character of information to be displayed on the screen, the combinational logic may result in substantial video signal propagation delay and adversely impact the synchronization of the video signal with other signals. In high resolution monitors having relatively short scan times (such as 50 nanoseconds for each dot of a character), the introduction of delays in the propagation of the video signal may require the addition of yet more logic in order to generate the video signal in a timely fasion.
The instant invention is directed to achieving an improved apparatus for generating modified video signals in response to multple video attributes associated with each individual character of information that is displayed on the display screen in a manner which will satisfy all synchronization requirements of the application and will result in substantial reduction in manufacturing costs.